Garnet Group Composition, Crystallization & Structure

    Composition. The garnets are orthosilicates which conform to the general formula Ra3" R2 “(Si04)3. R” may be calcium, magnesium, ferrous iron and manganese; R/// may be aluminum, ferric iron and chromium. The formulas of the chief varieties are given below; many of them, however, grade°lp.ore or less into each other.

    • Grosularite, Ca3:Al2(SiO4)3.
    • Pyrope, Mg.A12(Si04)3.
    • Almandite, Fe3.Al2(Si04)3.
    • Spessartite, Mn3Al2(Si04)3..
    • Andradite, Ca3Fe2(Si04)3
    • Uvarovite, Ca3(Cr,Al)2.(Si04)3.

    Crystallization. Isometric. Common forms dodecahedron (Fig. 311) and trapezohedron (Fig. 312), often in combination (Figs. 313 and 314). Hexoctahedron observed at times (Fig. 315). Other forms rare.

    Structure. Usually distinctly crystallized also in rounded grains; massive granular, coarse or fine.

    Physical Properties. H. = 6.5-7.5. G. = 3.15-4.3, varying with the composition. Luster vitreous to resinous. Color varying with composition; most commonly red, also brown, yellow, white, green, black. White streak. Transparent to almost opaque.

    Tests. With the exception of uvarovite, all garnets fuse at 3 to 3.5; uvarovite is almost infusible. The iron garnets, almandite and andradite, fuse to magnetic globules. Spessartite when fused with sodium carbonate gives a bluish green bead (manganese). Uvarovite gives a green color to snit oil phosphorus bead (chromium). Andradite is somewhat difficulty Voluble in hydrochloric acid and gelatinizes imperfectly on evaporation. All the other garnets are practically insoluble in acids. All oí them, with the exception oí uvarovitc, may be dissolved in hydrochloride acid after simple fusion and the solutions will gelatinize on evaporation. Garnets are usually recognized by their eharaeteristie isometric crystals, their handless, color, ete. It frequently rcquil'C8 an analysis to positively distinguish between the difficult members of the group.

    Varieties.

    • Grossularite, Essonite, Cinnamon Stone. Caleiumaluminum garnet. Often contains ferrous iron replaying calcium and ferric iron replacing aluminum. Color white, green, yellow, cinnamon-brown, pale red. Name derived from the botanical name for gooseberry, in allusion to the light green color of the original grossularite.
    • Pyrope. Precious garnet part. Magnesium-aluminum garnet Calcium and iron also present. Color deep red to nearly black. Often transparent and then used as a gem. Name derived from Creek, meaning firelike. Rhodolite is name given to a pale rose-red or purple garnet, corresponding in composition to two parts of pyrope and one of almandite.
    • Almandite. Precious garnet in part Common garnet in part z |Iron-aluminum gannet. Ferric iron replaces aluminum and magnesium replaces ferrous iron. Color fine deep red, transparent in precious garnet; brownish red, translucent to opaque in common gamete Name derived from Alabama, where in ancient times garnets were cut and polished.
    • Spessartite. Manganese-aluminum garnet. Ferrous iron replaces the manganese and ferried iron the aluminum. Color brownish to garnet-red.
    • Andradite. Common garnet in part Calcium-iron garnet.
      Aluminum replaces the ferric iron¡ ferrous iron, manganese and sometimes magnesium replace the calcium. Color various shades of yellow, green, brown to black. Named after the Portuguese mineralogist, d' Andrada.
    • Uvarovite. Calcium-chromium garnet. Color emerald-green. Named after Count Uvarov.

    Occurrence. Garnet is a common and widely distributed mineral, occurring as an accessory constituent of metamorphic and sometimes of igneous rocks. !ts most characteristic occurrence is in mica-schists, hornblende schists and gneisses. Found in pegmatite veins, more rarely in granite rocks. Grossularite is found chiefly as a product of contact or regional metarnorphism in crystalline limestones. Almandite is especially characteristic of the mica-schists. Pyrope is often found in peridotite rocks and the serpentines derived from them. Spessartite occurs in the igneous rock, rhyolite. Melanite, a black variety of andradite, occurs mostly in certain eruptive rocks. Uvarovite is found in serpentine associated with chromite. Garnet frequently occurs as rounded grains in stream- and sea-sands.

    Almandite, of gem quality, is found in northern India, Ceylon, Brazil, etc. Fine crystals, although for the most part too opaque for cutting, are found in a mica-schist on the Stikine River, Alaska. Pyrope of gem quality is found associated with clear grains of chrysolite (period) in the surface sands near Fort Defiance, close to the Utah-Arizona state line. Famous localities for pyrope gems are near Meronitz, Bohemia. Grossularite is only a little used in jewelry, but essonite or cinnamon stones of good size and color are found in Ceylon. A green andradite, known as demantoid, comes from the Urals and yields fine gems known as Uralian emeralds.

    Alteration. Garnet often alters to other minerals, particularly tale, serpentine and chlorite.

    Name. Garnet is derived from the Latin granatus, meaning like a grain. Carbuncle, an old name for garnet and other red stones, was derived from the Latin word carbo, coal, and is used at present to designate garnets cut in oval form

    Use. Chiefly as a rather inexpensive gem stone. Sometimes ground and used on account of its hardness for abrading purposes, as sand for sawing and grinding stone, or for making sandpaper.